1. Field of the Invention
Embodiments of the invention relate to internal RF antennas for use with ion plasma sources. More particularly, the present invention relates to internal RF antennas for use with ion plasma sources that include a dielectric insulation component.
2. Discussion of Related Art
Plasmas are used in a variety of ways in semiconductor processing to implant wafers or substrates with various dopants, to deposit or to etch thin films. Such processes involve the directional deposition or doping of ions on or beneath the surface of a target substrate. Generally, plasmas are generated by supplying energy to a neutral gas introduced into a chamber to form charged carriers which are implanted into or deposited on the target substrate. The gas is ionized by any of several methods of plasma generation including, but not limited to DC glow discharge, capacitively coupled RF, inductively coupled RF, etc.
An RF source may have an antenna positioned external a dielectric window of an ion source chamber or have an antenna positioned inside the ion source chamber. In an RF source having an internal antenna, an induction coil or RF antenna is internally disposed in the ion source chamber and supplies energy to ionize gas introduced into the chamber. The RF antenna within the chamber usually takes the shape of a coil which includes power leads that extend through seals in the walls of the chamber. Since these RF antennas are exposed to the generated plasma within the ion source chamber, the antenna is usually protected by a dielectric coating such as glass, porcelain, alumina, etc. However, coated RF antenna coils generally have limited lifetimes because the coatings typically contain micro-pores or hair fractures that lead to punctures in the dielectric coating material when exposed to caustic RF plasma material. Alternatively, a glass tube may be used to insulate the antenna. These tubes have a shape corresponding to the conductive material being encased through a process of heating and bending. Since glass has a high melting point, the conductive metal must also have a high melting point to make the materials malleable enough to bend into the desired shape. Two often used metals are stainless steel and titanium. Stainless steel has a resistivity of approximately 69-75×10−8 Ω-m while titanium has a resistivity of approximately 42×10−8 Ω-m. Conductors with high resistivity lead to a large Q factor which generally refers to the quality factor of the transmitter coil. Conductors such as copper (1.7×10−8 Ω-m), aluminum (2.65×10−8 Ω-m), and silver (1.65×10−8 Ω-m) have lower resistivity as compared to stainless steel and titanium. Thus, the Q factor for titanium and stainless steel having high resistivity stainless steel and titanium is approximately 25-45 times lower as compared to a material such as copper which has a relatively lower resistivity metal such as copper. A material that has a high Q (e.g. stainless steel, titanium) antenna will be much less efficient and will suffer inductive losses as compared to a lower Q (copper, aluminum, silver) antenna. In addition, if the melting points of the glass and the metal conductor are too dissimilar, one may liquefy or become too malleable with respect to the other. The problem with high melting point metals is that they have higher electrical resistance properties which lead to an inefficient high Q for an RF antenna.
Some RF antennas may employ a conductive metal such as copper, but must then use a lower grade glass having a much lower melting point than quartz to maintain an acceptable range for the melting points of both materials. The use of a lower grade glass, however, introduces other unwanted characteristics such as sputtering that can lead to contamination issues and electrical shorting of the antenna to walls of the chamber. Extensive sputtering may eventually puncture the antenna casing (lower grade glass) which may lead to water leaks and flooding of the vacuum chamber of the ion source. It is with respect to these and other considerations that the present improvements have been needed.